The air drying properties of sorbents such as silica gel, lithium chloride, and alumina are well known and have been used in many industrial applications. However, the applicability of these sorbents, and in particular silica gel, as a desiccant in an air conditioning system has been explored only recently.
One of the pioneering systems was the Munters Environmental Control (MEC) System developed at the Institute of Gas Technology (IGT). In that system a rotating matrix, consisting of a series of channels whose walls are made of a sorbent, exchanges moisture and heat with a stream of air flowing through it. The MEC concept has been known for about thirty years. The first patent covering this principle was issued in 1949. Despite efforts by both the U.S. and European organization over the years, the concept was never successfully developed to a commercial reality. IGT became interested in the potential advantages of MEC about thirteen years ago. It applied modern engineering analysis and computer modeling techniques to study the system. IGT's wholly owned subsidiary, Gas Developments Corporation, gained a license for the patent from A. B. Carl Munters of Sweden, the owner of the patent. The IGT modifications included an asbestos-wax, heat-transfer wheel and an asbestos-lithium chloride drying wheel.
Another variation, MEC II, used an aluminum heat transfer wheel to improve heat transfer efficiency. The aluminum wheel, however, became hygroscopic with time, absorbing water and transferring it to the conditioned dry air stream. Also, the lithium chlorine on the drying wheel deteriorated into other compounds that could not carry out the drying function.
In another variation, MEC III, constructed in 1973, the aluminum wheel was coated with a proprietary material that reduced the water carry-over to an acceptable level. The drying wheel used a newly perfected molecular sieve absorbent material. The new material was a paper-thin asbestos sheet carrying over 50% molecular sieve. The sheet was corrugated and formed into a wheel. Later that device was adapted to use solar heat and a natural gas boost to make up for the solar heating deficiency.
Pennington (U.S. Pat. No. 2,700,537) describes a humidity changer for air conditioning that uses a rotary moisture transferrer packed with an inert, air-pervious carrier having a rigid space structure, and impregnated with a liquid sorbent.
Another wheel-type humidifier was built by Cargocaire under the brand name "HoneyCombe." In that device a wheel core was made of a non-metallic, non-corrosible, bacteriostatic, inert structure impregnated with an inorganic, non-granular, crystalline, particle solid desiccant which transfers water in the vapor phase. The desiccant was evenly dispersed throughout the microscopic pores of the wheel structure. The wheel structure consisted of small flutes or tubes parallel to the axis of flow, allowing laminar air flow to give the maximum moisture transfer with minimum friction loss. Humid air passing through the flutes was dried. Simultaneously, a counterflowing hot reactivation air stream passed through the flutes in the reactivation sector to remove the moisture picked up by the desiccant thus assuring continuous controlled drying.
The capacity of such exchangers is generally reduced due to the large heat effects associated with sorption of water. This is because at a given humidity the equilibrium capacity of a sorbent decreases with temperature. There is another problem resulting from this reduced capacity at higher temperatures. After regeneration the matrix, in which the hygroscopic salt is held, is left at a high temperature. During sorption, although the relatively cooler incoming air cools the matrix down, the heat effect associated with sorption may not allow the desired level of humidity in the outgoing air. As a remedy, cooling of the solid during sorption has been suggested to obtain the desired temperature and humidity levels. This can be accomplished by using a cross-flow heat exchanger where the cooling and the process streams are separated by a solid wall; and where the cooling and process streams flow perpendicular to each other.
Among the common desiccants used for drying air silica gel has the unique property of showing a sharp decrease in the equilibrium sorptive capacity with a temperature increase at a given partial pressure of water over it. Although silica gel can be regenerated at lower temperatures than other desiccants (such as molecular sieves or activated alumina), the immediate disadvantage arising from a sharp decrease in capacity with an increase in temperature is that in an adiabatic rotary dehumidifier (with alternate sorption and desorption) the desired humidity level may not be achieved during sorption. If with the help of cross-cooling, the desired humidity level can be achieved, silica gel would then become a desirable desiccant in a rotary dehumidifier. Cross-cooling would delay the "break-through" time for drying operations thus allowing a slower speed of rotation for a rotary exchanger than for the corresponding adiabatic case.
Alternatively, for a given breakthrough time and process channel width, channel length would be reduced to cross-cooling. More importantly, there is an accompanying reduction in volume occupied by the process channel compared to the adiabatic case. However, part of this advantage is lost due to the increased volume accompanying the addition of cooling channels.
A quantitative study of a cross-cooling dehumidifier was performed and reported in Chemical Engineering Science 1974, volume 29, pages 2101 through 2114. That study showed that cross-cooled dehumidifiers can be smaller and require less power than corresponding adiabatic exchangers. Those calculations also showed that regeneration temperatures below 180.degree. F. in conjunction with cross-cooling result in sufficient dehumidification for air conditioning applications.
A solar powered dehumidifier was proposed by the Energy Research and Development Authority (ERDA) under its Solar Activated Cooling (SAC) project. It was recognized that solar energy in the form of heat could be used to dehydrate the desiccant thus closing the functional cycle.